At least since Woodrow Wilson, prominent Americans have wondered whether we would be better off with a parliamentary rather than a Presidential government. Wilson's own views evolved during the course of his career so that he eventually took the relative vigor of his predecessors in office as a model for how American government could operate effectively. Nonetheless, even if one ends up where Wilson did, the case for separation of powers rather than parliamentarianism is at best a matter of tallying costs and benefits.
The costs are substantial. Here is the early Wilson in Congressional Government, warning about the paralysis to which divided government can lead:
[I]t is impossible to deny that this division of authority and concealment of responsibility are calculated to subject the government to a very distressing paralysis in moments of emergency. There are few, if any, important steps that can be taken by any one branch of the government without the consent or coöperation of some other branch. Congress must act through the President and his Cabinet; the President and his Cabinet must wait upon the will of Congress. There is no one supreme, ultimate head—whether magistrate or representative body—which can decide at once and with conclusive authority what shall be done at those times when some decision there must be, and that immediately. Of course this lack is of a sort to be felt at all times, in seasons of tranquil rounds of business as well as at moments of sharp crisis; but in times of sudden exigency it might prove fatal,—fatal either in breaking down the system or in failing to meet the emergency. Policy cannot be either prompt or straightforward when it must serve many masters. It must either equivocate, or hesitate, or fail altogether.What are the benefits of our system? The chief one is supposed to be that separation of powers acts as a check on tyranny. Because power is divided, it is more difficult for a tyrant to consolidate power. That's the theory anyway, but I don't think it works that way. I haven't done the accounting, but I would be surprised if it turned out that over the course of history, parliamentary systems more often devolved into tyranny than did Presidential ones. Everyone immediately thinks of Weimar Germany but there are numerous examples of Presidential systems--especially in Latin America--falling prey to coups. Indeed, one can tell a story in which this is the logical result: a President with a power base that is independent of the legislature will be able to justify his gradual accretion of powers or can point to the ineffectiveness of the legislature as the immediate justification for seizing power.
In any event, mature democracies--whether they are parliamentary, Presidential, or mixed--seem largely stable, so the fear of autocracy seems misplaced here. The real cost-benefit analysis is more prosaic. To paint with a very broad brush, if you think that the aggregate risks of government making changes to the status quo are greater than the risks of doing nothing, you will favor a separation-of-powers regime like our own. Conversely, if you think that on average, the risks of doing nothing are greater than the risks of bad government action, you will favor a parliamentary system.
Certainly in the current situation, it is easy to see how our Constitution is contributing to the basic problem, because the greater danger is posed by inaction. If we had a fully Democratic Congress (including a large enough majority in the Senate to override a filibuster), the debt ceiling would be raised. Ditto if we had a fully Republican Congress and a Republican President. Neither party would want to be clearly on the hook for the economic disaster that would follow. But with divided government, we get brinksmanship that could actually lead to default, which anyone with any sense at all will agree is a terrible outcome. We could end up there nonetheless because divided government means that no one knows for sure who will be blamed. Or as Wilson put it: "If there be one principle clearer than another, it is this: that in any business, whether of government or of mere merchandising, somebody must be trusted, in order that when things go wrong it may be quite plain who should be punished."